Allograft bone graft substitutes for spine fusion surgery

The biological and biomechanical environment for spine fusion surgery varies in different parts of the spine. The key factors are:

1. Whether the spine fusion surgery is done in the cervical, thoracic or lumbar spine, and;
2. Whether the anterior (front) or posterior (back) spine is chosen as the intended fusion site.

These factors are the major determinants of the usefulness of allograft (cadaver bone obtained from a tissue bank) as a replacement for autograft (the patient’s own bone) in spine fusion surgeries. Allograft bone is an effective bone graft substitute in the pediatric population for scoliosis surgery, but is only useful as a bone graft supplement posteriorly in the adult patient.

In the cervical spine, allograft is reasonably effective anteriorly in single and possibly two-level applications, depending if an anterior cervical plate is also used.

• In the thoracic spine, structural allograft is frequently used anteriorly.

• In the lumbar spine, autograft is traditionally considered the “gold standard” for posterior fusions. Most elective spinal fusions are performed in the lumbar spine, and this area is also the most difficult part of the spine to fuse successfully. Anterior structural allograft bone is well accepted in the anterior spine to support the interbody space.

• Both anterior and posterior lumbar fusions continue to have an important role in the treatment of degenerative spine conditions. The anterior lumbar spine is widely considered to offer a superior biological and biomechanical environment for fusion due to the compressive loads transmitted to the anterior structural graft.

In the lumbar spine, fusing the anterior column of the spine by removing the intervertebral disc places the bone graft under compression due to the biomechanical forces supporting the upper body. Bone in compression tends to fuse more readily than bone in tension.

Also, and perhaps even more importantly, with this approach the extensive blood supply of the vertebral bodies provides a rich supply of cells, growth factors and other nutrients needed to make bone grow. Small holes made within the vertebral endplates allow access of bone marrow elements from the host vertebral bodies to the interbody graft, providing nutrients for bone healing.

Access to the anterior spine may be obtained via an incision along the abdomen (anterior approach) or the back (posterior approach). There are technical differences between the two techniques for fusing the disc space. The end result is the attainment of a stable fusion through the addition of a grafting material.

If fusion is performed via an incision in the back to address the posterior spine (transverse processes, facets), the fusion bed is placed under tension and must heal in a non-ideal biomechanical environment. Also, the posterior spine may eventually be much less vascular following muscular manipulation and scarring than the anterior spine, so fusion is theoretically more difficult to achieve. However, posterior lumbar spine fusion is a less technically demanding procedure and offers certain advantages in some patients, and it is an option to be seriously considered.

Allograft is available in a variety of forms for various applications. Most spinal fusions employ autograft bone ground up (morselized) into small pieces that stimulate cells to produce the growth factors that cause bone to grow. It also contains the necessary calcium scaffolding.

Methods for processing allograft bone have produced a variety of products, such as demineralized bone matrix (DBM), which contains concentrated growth factors. These products have performed well in certain spine fusion applications. However, all forms of allograft carry a remote risk of disease transmission, therefore alternatives will continue to be sought.